Fertilizer is defined as any material added to a soil to supply plant nutrients. Commercially produced fertilizers account for much of today's crop yields. Fertilizers can be broadly classified as organic fertilizers (e.g., of plant or animal origin) and inorganic fertilizers (synthesized from chemicals). Inorganic fertilizer is accredited with supporting much of the recent global population growth. Indeed, almost half of the people on earth are currently fed by crops grown using synthetic nitrogen fertilizers, and this number is expected to increase for coming generations.
Inorganic fertilizer can be synthesized using the Haber process, which produces ammonia. Ammonia is used as a feedstock and oxidized to produce nitrates and nitrites, which can then be used to produce nitrate-based fertilizer, such as anhydrous ammonia, ammonium nitrate, and urea.
The Haber process harnesses nitrogen in the atmosphere to produce ammonia. In an example, a hydrogen source (e.g., methane gas) is reacted with steam to produce carbon dioxide, steam, and hydrogen. Ammonia can then be synthesized using an iron catalyst at a pressure of about 150-250 bar and a temperature between about 300 and 550° C. A catalyst may be used with multiple cycles to achieve conversion efficiencies greater than 90%. Conversion of atmospheric nitrogen and hydrogen gas to ammonia proceeds according to the following generic chemical formula:N2(g)+3H2(g)→2NH3(g)
While the Haber process is responsible for meeting many of the world's fertilizer needs today, the process is energy intensive. That is, the Haber process requires large amounts of energy to produce nitrogen, increasing the cost of production as carbon-based energy continues to increase in cost. Other industrial processes are available for producing other chemical compounds (e.g., hydrocarbons). But these industrial processes also suffer from high energy input requirements.